1. Field of the Invention
This invention relates to an EGR control system for an internal combustion engine.
2. Description of the Related Art
In internal combustion engines, EGR (Exhaust-Gas Recirculation) control for recirculating part of the exhaust gas is conducted in order to improve fuel economy and reduce exhaust gas pollutants.
Recent years have seen the development of direct-injection spark ignition internal combustion engines where gasoline fuel is injected directly into the combustion chamber to achieve lean stratified-charge combustion. EGR control has also been applied to this type of engine. An example can be found in Japanese Laid-open Patent Application No. Hei 9 (1997)-32651.
When a direct-injection spark ignition engine is operating in the low engine speed and low engine load region, gasoline fuel is injected during the compression stroke to cause stratified-charge combustion (ultra-lean burn combustion) at an air-fuel ratio of, for instance, 30:1 or greater. When the engine is operating in the high engine speed and high engine load region, gasoline fuel is injected during the intake stroke to cause premix-charge combustion (uniform combustion) at an air-fuel ratio of, for instance, 20:1 or less.
In the stratified-charge combustion region, the EGR amount (or rate) should preferably be increased to reduce the NOx (nitrogen oxides) content. As can be seen from FIG. 27, if the EGR amount is increased, stratified-charge combustion does not fluctuate greatly and remains stable, since the marginal limit of EGR is high owing to the stratification. In the premix-charge combustion region, however, combustion grows increasingly unstable with increasing EGR amount. Thus, the marginal limit of EGR is lower than in the stratified combustion region and the required EGR amount is relatively small.
Viewing this from a different point, as shown in FIG. 28, a large amount of EGR gas is required for NOx reduction in the stratified-charge combustion region. However, introduction of EGR gas is difficult, since the pressure difference between the intake air and the exhaust gas becomes small when the engine operates with full-throttle.
In the premix-charge combustion region, on the other hand, the pressure difference between the intake air and the exhaust gas is sufficient, since the engine load is regulated through the throttle opening, similarly to the case of an ordinary engine where gasoline fuel is injected before the intake valve(s). As shown in FIG. 27, however, increasing the EGR amount destabilizes combustion and the margin of EGR is therefore not high. This means that the diameter or capacity of an EGR control valve need be only about the same as that in an ordinary engine where fuel is injected before the intake valve(s).
It can thus be seen that the marginal limit of EGR differs between the stratified-charge combustion region and the premix-charge combustion region in the direct-injection spark ignition engine. However, the combustion mode must frequently be switched between the stratified-charge combustion and the premix-charge combustion in response to the engine operating conditions.
Therefore, if the characteristic of the EGR control valve is designed or set with the focus on the premix-charge combustion region where the marginal limit of EGR is relatively low, then, as shown in FIGS. 29A and 29B, the EGR control valve response is deficient when the combustion mode is switched to the stratified-charge combustion in response to a change in the engine operating condition. The EGR amount is therefore insufficient. On the other hand, if the characteristic of the EGR valve is designed or set with focus on the stratified-charge combustion region, the EGR control valve response becomes too high when the combustion mode is switched to the premix-charge combustion. The EGR amount therefore becomes excessive.
As shown in FIG. 29C, the deficient/excessive EGR amount destabilizes combustion to cause misfiring and degraded drivability, and further, as shown in FIG. 29D, increases unburnt HCs (hydrocarbons) that degrade emission performance. While designing the EGR amount low prevents misfiring etc., this expedient is undesirable, since it makes full utilization of the expected engine performance impossible and is also disadvantageous in terms of fuel economy.
The technique proposed in the aforesaid prior art of coping with these problems is to drive the EGR control valve at a high speed when the combustion mode is switched from the stratified-charge combustion to the premix-charge combustion and to drive it at a lower speed when the combustion mode is switched from the premix-charge combustion to the stratified-charge combustion.
However, the aim of this prior art technique is to achieve exhaust gas purification by conducting EGR control in the stratified-charge combustion region and the premix-charge combustion region, and to prevent engine output deficiency and to lower torque shock when the combustion mode is switched, while preventing transient deterioration of combustion when the EGR amount (or ratio) is changed. Specifically, in this prior art, the purpose in increasing the EGR valve driving speed when the combustion mode is switched to premix-charge combustion is to improve response to demand for increased engine output without causing combustion deterioration. And the purpose in decreasing the EGR driving speed when the combustion mode is switched to the stratified-charge combustion is to avoid combustion deterioration at the time of transition.
It has also been proposed in this prior art to make the driving speed of the EGR control valve higher in the closing direction than in the opening direction. However, the principle involved in this prior art is the same as that just explained.
In other words, the prior art is limited to changing the driving speed of the EGR valve in response to the combustion mode and does not propose an improved EGR mechanism for an engine with different combustion modes that is responsive to the combustion mode for realizing an EGR amount that is neither deficient nor excessive.